Efficient Organocatalytic Dehydrogenation of Ammonia Borane

Abstract Dehydrogenation of ammonia borane by sterically encumbered pyridones as organocatalysts is reported. With 6‐tert‐butyl‐2‐thiopyridone as the catalyst, a turnover frequency (TOF) of 88 h−1 was achieved. Experimental mechanistic investigations, substantiated by DLPNO‐CCSD(T) computations, indicate a mechanistic scenario that commences with the protonation of a B−H bond by the mercaptopyridine form of the catalyst. The reactive intermediate formed by this initial protonation was observed by NMR spectroscopy and the molecular structure of a surrogate determined by SCXRD. An intramolecular proton transfer in this intermediate from the NH3 group to the pyridine ring with concomitant breaking of the S−B bond regenerates the thiopyridone and closes the catalytic cycle. This step can be described as an inorganic retro‐ene reaction.

The controlled release of dihydrogen from ammonia borane (AB) with its H 2 content of 19.7 wt %i so fi nterest considering its potential use as ah ydrogen-storage material. [1] Several transition metals catalyze the dehydrogenation of AB efficiently. [2] Among the most effective catalysts are nickel carbene complexes and noble transition-metal complexes with pincer-type phosphine ligands,b ut iron pincer complexes have also proved to be effective. [3][4][5] Since Wegner and co-workers showed that this reaction can also be catalyzed by ab identate Lewis acid, the dehydrogenation of AB by main group systems has attracted considerable attention. [6,7] Slootweg,Uhl, and co-workers reported aphosphine/aluminium-based frustrated Lewis Pair (FLP) that effects the stochiometric dehydrogenation of AB and the catalytic dehydrogenation of dimethylamine-borane (DMAB). [8] Aldridge et al. showed that ax anthene-based FLP catalyzes hydrogen release from AB and provided evidence for ac hain-growth mechanism. [9] However,t he reported turnover frequencies (TOF) of 4h À1 are moderate compared to transition-metal catalysts.E arlier this year,t he field was advanced by the report that ag eometrically constrained phosphine-borane FLP displays improved activity for the dehydrogenation of DMAB,b ut the catalyst showed only moderate activity regarding dehydrogenation of AB. [10] Forp ractical applications,a ne fficient and easily accessible organic catalyst is desirable.D ixon and co-workers showed that strong Brønsted acids initiate the dehydrogenation of AB,presumably by protonation of the hydridic B À Hgroup. [11] We thus envisioned that an organic molecule possessing an acidic group and abasic site could serve as an organocatalyst for the dehydrogenation of AB by protonation of the BH 3 group and deprotonation of the NH 3 group (Scheme 1).
This organocatalyst would have to be able to revert to its initial form in order to form ac atalytic cycle.2 -Hydroxypyridine satisfies the criteria of an acidic OH group and ab asic pyridine ring. Furthermore,t he tautomers 2-pyridone and 2hydroxypyridine are almost isoenergetic.W et herefore considered 2-pyridone as as uitable candidate for the catalytic dehydrogenation of AB.Aside from simple 2-pyridone 1,the sterically more encumbered 6-tert-butyl-2-pyridone (2)w as tested as ac atalyst. Furthermore,t he more acidic thiopyridones 3 and 4 were used. [12] We attempted the dehydrogenation of AB by reacting 1mol %o ft he respective organocatalyst with AB at reflux in THF (Scheme 2). Theresults of the catalytic reactions are summarized in Table 1.
Thep arent pyridone 1 shows only moderate catalytic activity.However,1mol %ofthe sterically more encumbered Scheme 1. The working hypothesis of this research project:Dehydrogenation of AB by an organocatalyst through simultaneous protonation and deprotonation.
Scheme 2. Dehydrogenation of AB by various pyridine derivatives.
6-tert-butyl-2-pyridone (2)c atalyzes hydrogen release from AB with an otably higher efficiency: 0.6 equivalents of hydrogen were liberated within 2h,w hich corresponds to aTOF of 31 h À1 .Borazine is the main product of this reaction, as demonstrated by 11 BNMR. Thiopyridone 3 is less active than 2 but displays as lightly higher activity than parent pyridone 1.This result indicates that the combination of steric demand and increased acidity should lead to an active catalyst. Indeed, 1mol %6 -tert-butyl-2-thiopyridone (4)c atalyzes the liberation of 1.8 equiv H 2 from AB within 2h, which corresponds to aT OF of 88 h À1 .T his is,t othe best of our knowledge,hitherto the highest TOFfor H 2 release from AB reported for ametal-free system. Analysis of the reaction mixture shows that AB is completely converted into borazine and polyborazylene.
With these unexpected results in hand, we aimed for amechanistic understanding regarding the mode of action by which 4 catalyzes hydrogen release from AB.T overify that 4 does not act as aBrønsted acid and initiates the dehydrogenation of AB through ac hain-growth mechanism, ac atalytic reaction using 1mol %thiophenol (which is more acidic than thiopyridone), was performed. This reaction led to the formation of B-(cyclotriborazanyl)-amine-borane as the main product (Scheme 4). Theo bserved TOF of 27 h À1 is significantly lower than that achieved with 4 as acatalyst. This corroborates the importance of catalyst bifunctionality,t hat is,t he presence of the basic pyridine ring for the catalytic activity of 4.I ti st empting to attribute the higher activity of the tert-butyl derivatives 2 and 4 to the destabilization of their respective dimers.T he synthesis of 4 has been described previously,b ut its SCXRD structure has not been reported yet. [13] Single crystals suitable for X-ray analysis were obtained in the course of this study. [14] TheSCXRD structure is that of the thiopyridone dimer 4 2 (Figure 1). TheN ÀH···S distance of 3.46 is elongated by 0.17 compared to the C 2h symmetric dimer of 3. [15] Thef ormation of monomeric 4SH that is assumed to be the active catalyst was further investigated computationally at the SMD(THF)-TightPNO-DLPNO-CCSD(T)/def2-QZVPP//PBE0-D3(BJ)/def2-TZVP level (Figure 2). [17,18] Tautomerization of 4 2 requires an activation energy of 15.8 kcal mol À1 .T he formation of 4SH from 4 2 is slightly endergonic.I nc omparison, the formation of 3SH from 3 2 is thermodynamically disfavored by 5.2 kcal mol À1 .T his result indicates that aground-state effect, that is the destabilization of 4 2 ,c ontributes to the activity of 4.
We then focused our attention on the detection of potential reactive intermediates.Upon monitoring astoichiometric reaction of 4 with AB at 60 8 8CbyNMR, the formation of the mercaptopyridine-borane complex 5 was observed within 5h.The NH 3 group of 5 gives rise to acoalescent signal at 5.48 ppm in the 1 HNMR spectrum. TheBH 2 group shows as ignal at 2.62 ppm that integrates to two.Atriplet at   À13.3 ppm is observed by 11 BNMR, which is atypical shift for at etracoordinated borane. [19] AN OE contact detected by NOSY NMR confirms spatial proximity between the NH 3 group and the tert-butyl group of the thiopyridone.Attempts to isolate 5 from solution were not successful. However,upon reaction of 4 with DMAB,astable surrogate of 5 was obtained. Them olecular structure of this surrogate 5 Me2 , derived from SCXRD,supports the structural assignment of 5 ( Figure 2). Thes tructure shows as hort N(H)···N hydrogen bond that indicates the possibility of an intramolecular proton transfer to the pyridine ring. It is reasonable to assume that 5 is the product of ad ehydrogenative coupling between the mercaptopyridine form of 4 and AB.T hat implies that the dehydrogenation of AB commences with this dehydrogenative coupling,w hich liberates the first equivalent H 2 and yielding 5.
When NH 3 BD 3 was used as the substrate in the catalytic reaction, ak inetic isotope effect (KIE) of 1.20 AE 0.15 was observed. This result is consistent with the computed transition state for the dehydrogenative coupling:W hile the SÀHbond is ruptured, the BÀHbond is only slightly distorted ( Figure 3). [20] Indeed, the computed KIE for the dehydrogenative coupling of 1.01 agrees favorable with experimentally observed KIE. [21] As econd AB molecule is required to stabilize the partial negative charge on the thiolate in the transition state.
Upon prolonged heating of asolution of 5,the formation of borazine and regeneration of 4 was observed (Scheme 5). Ther eactivity of 5 was further investigated computationally ( Figure 4). Proton transfer from the NH 3 group to the pyridine ring and the concomitant breaking of the SÀB bond requires af ree activation energy of 11.0 kcal mol À1 . [22] This result indicates that 5 is not inert at 60 8 8C. However,the liberation of NH 2 BH 2 and the regeneration of 4 are endergonic.T herefore, 5 can be observed in as toichiometric reaction since it is thermodynamically stable with respect to the formation of NH 2 BH 2 and 4.The fact that 5 does react to borazine and 4 at elongated reaction times further indicates that the formation of borazine renders the stoichiometric reaction exergonic (Scheme 5). [23] We note that, regarding the

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Communications 1592 www.angewandte.org reorganization of p-electron density,l iberation of NH 2 BH 2 from 5 can be described as an inorganic retro-ene reaction.
Further evidence that the retro-ene reaction is part of the catalytic cycle came from astoichiometric experiment with 4 and ND 3 BH 3 .U pon reaction at elevated temperatures,t he formation of borazine and incorporation of deuterium in 4 is observed (Scheme 6). Ap ronounced KIE of 2.4 AE 0.3 is observed when ND 3 BH 3 is used as the substrate in the catalytic reaction. Given the low barrier for the retro-ene reaction, this KIE is presumably due to the deuterium incorporation in 4.I ndeed, the computed KIE for the dehydrogenative coupling (see Figure 3) starting from deuterated 4SD and ND 3 BH 3 is 3.2, which is in reasonable agreement with the experimentally observed KIE.
Based on the experimental and computational investigations,w ep ropose am echanism for the dehydrogenation of AB by 4 that commences with tautomerization of 4 to the mercaptopyridine form 4SH, presumably via its dimer (Scheme 7). Ad ehydrogenative coupling of AB with monomeric 4SH yields borane 5.T he liberation of NH 2 BH 2 regenerates monomeric 4,w hich dimerizes and completes the catalytic cycle.H owever,c ontributions from an acidinduced chain-growth mechanism in the dehydrogenation of AB catalyzed by 4 cannot be excluded.
In summary,w ehaved ocumented that hydrogen release from AB is efficiently catalyzed by 6-tert-butyl-2-thiopyridone.M echanistic investigations highlight the importance of bifunctionality of thiopyridone for the catalytic activity,while the tert-butyl group facilitates the monomerization of 4.T he results reported herein are likely to stimulate the development of efficient organocatalysts for hydrogen-storage applications. [24]